1. Technical Field of the Invention
The present invention relates to a resin molded article for optical products comprising a copolymer predominantly composed of methyl methacrylate and a styrenic monomer, and to a light transmitting plate comprising the same.
2. Description of Related Art
A methyl methacrylic polymer is a thermoplastic resin, which has a visible light transmittance higher than that of glass and excellent optical properties and is of low birefringence. Therefore, it has long been used as a material for a wide variety of optical products, such as optical lenses, optical discs, light transmitting plates for backlight of liquid crystal display devices, and others.
Particularly, in recent years, there is an increasing demand for its use as a light transmitting plate for backlight of a liquid crystal display device. The light transmitting plate for backlight is what has the functions of propagating and diffusing light entered from a certain direction and having the light exit in the direction of liquid crystal cell. Liquid crystal display devices is required to be thinner, therefore, an edge-lighting system is generally adopted, in which a light source is placed adjacent to an edge of the light transmitting plate.
The amount of light which is lost when traveling through the light transmitting plate increases in the edge-lighting system, since the distance in the light transmitting plate traveled by the light is relatively long. For preventing the light loss, the material of the light transmitting plate is required to have a high light transmittance. For such reason, methyl methacrylic polymers, which are excellent in terms of optical properties, have been conventionally employed for use in light transmitting plates.
However, a light transmitting plate made of a methyl methacrylate polymer has the problem of high moisture absorbency, which tends to cause dimensional changes or warp. Meanwhile, as described above, since a thinner liquid crystal display device has been demanded, even if attempting to provide the liquid crystal display device with a backlight, there is left almost no place between the light transmitting plate and a diffusing film and a reflecting sheet that are positioned on the upper and lower surfaces of the light transmitting plate respectively. Therefore, even its warp is slight, the light transmitting plate is brought into tight contact with the diffusing film, resulting in white turbidity of the display screen at an area corresponding to where the plate and the film are in tight contact. Moreover, if the dimensions of the light transmitting plate are unstable, not only assemblage of a liquid crystal display will be hindered but also the quality of the resulting display will be adversely influenced.
The object of the present invention is to provide a resin molded article for optical products which is improved in hygroscopic property, less susceptible to warp or deformation, and of high light transmission, and a light transmitting plate comprising the same.
The present inventors made intensive studies to achieve the above-described object and finally found that a molded article comprising a methyl methacrylic copolymer containing a specific amount of a styrenic monomer, and being specific in foreign body content is less susceptible to warp and almost equal in visible light transmittance to an article of a methyl methacrylic polymer. The present invention was accomplished based on this finding.
That is, the present invention provides a resin molded article for optical products comprising a copolymer predominantly composed of methyl methacrylate and a styrenic monomer, which is characterized in that the number of foreign bodies contained in the resin molded article having a length of 210 mm, a width of 210 mm and a thickness of 3 mm as visually examined using a strain meter is 150 or less; and a light transmitting plate comprising the same.